1. Field of the Invention
The present invention relates to personal physiological monitors, and, in particular, to personal physiological monitors which detect changes in galvanic skin resistance, skin temperature, or both.
2. Description of the Prior Art
The desirability of continuously monitoring galvanic skin resistance and skin temperature is well-known. Changes in skin temperature at the extremities and the presence of perspiration on the skin surface (resulting in a decreased galvanic skin resistance) are symptomatic of various physiological conditions, such as, for example, the onset of hypoglycemia in a diabetic.
Hypoglycemia occurs in a diabetic when, for various reasons, the diabetic's blood glucose level falls below a certain value. If detected, the hypoglycemic state can readily be reversed by ingestion of orange juice or other common sources of digestable sugar. However, if undetected, the hypoglycemic diabetic enters a deep comatose state, which can result in severe brain damage or death.
A conscious adult diabetic can typically ascertain the onset of hypoglycemia by recognizing any of the various physical symptoms associated with a hypoglycemic condition, such as; a feeling of anxiety or nervousness, blurring vision, inability to focus the eyes, nausea, or unexplained perspiration. Once these symptoms are recognized, sugar can be ingested and the hypoglycemic onset can be reversed. However, if the hypoglycemic onset occurs and continues while the diabetic is asleep, or if the diabetic is, for a variety of reasons, unable to recognize such physical symptoms, he can enter a comatose state before the hypoglycemia is detected. Accordingly, since it is known that the onset of hypoglycemia is commonly evidenced by the presence of profuse perspiration, decreased skin temperature at the extremities, or both, such parameters can be monitored and alarm indicia can be generated in response to such external physical symptoms.
Typically, hypoglycemic onset is first evidenced by profuse perspiration, and subsequently by a decrease in skin temperature. Thus, decreased galvanic skin resistance, as a result of such perspiration, is typically the primary indicator in detection of hypoglycemic onset. However, diabetes tends to cause autonomic neuropathy, sometimes rendering a long-term diabetic incapable of perspiring (thereby degrading the reliability of perspiration as an indicator of hypoglycemic onset). Decreased temperature at the extremities, while typically occurring after the normal perspiration phase, tends to be a more reliable indicator of hypoglycemic onset, and is usually present in diabetics who lack the perspiration response.
In general, devices for measuring galvanic skin resistance are known. Typically, electrodes are disposed on the skin, and current passing between the electrodes is measured by a cooperating indicator device to provide indicia of the skin resistance. Typical indicator devices include analog meters calibrated in ohmic resistance, chart recorders, variable pitch or intensity sound sources, and lights.
Similarly, measurement of skin temperature is, in general, known. Skin temperature is typically sensed by a thermistor (temperature sensitive resistance) or similar device placed against the skin. Indicia of temperature is typically provided by a meter, variable tone source, or the like. In some cases a predetermined threshold or limit condition is established and an alarm sounded upon unfavorable comparison with such threshold condition.
Personal physiological monitors which generate an alarm indicia in response to changes in skin resistance, skin temperature, or both, are also known. An example of such a monitor is described in U.S. Pat. No. 4,178,916 issued to E. W. McNamara on Dec. 18, 1979. The McNamara device is adapted to be worn on the wrist of a diabetic. Skin resistance is sensed using respective electrodes disposed on the underside of the monitor casing the contact with the wearer's skin. A current is passed between the respective electrodes, and the voltage developed therebetween is compared to a fixed reference. In essence, an alarm indicia is generated when the skin resistance drops below a fixed predetermined value. Skin temperature is sensed utilizing a thermistor disposed on and projecting through the underside of monitor case to contact the wearer's skin. The thermistor is electrically connected in a bridge circuit with a manually set potentiometer. The potentiometer is typically set utilizing a thumbwheel. When the skin temperature drops below a threshold in accordance with the manually set potentiometer, alarm indicia are generated.
It is known that a monitor must generate alarm indicia capable of awakening even the deepest sleeper. However, space constraints inherent in a physiological monitor which can be worn on a human extremity impose limitations on the physical size of sound transducers which may be used, and on the size of any integral power supply available to the transducer. The McNamara patent proposes to solve such problem by radiating electromagnetic energy to a radio receiver in response to detection of an alarm condition. The radio would then generate a sound sufficiently loud to awaken a sleeper.
The prior art physiological monitors are disadvantageous in a number of respects. The constant current passed between the electrodes to sense skin resistivity constitutes a substantial power drain on the system, severely limiting the lifetime of battery cells in such monitor devices. Further, it has been found that the current flow between the respective terminals often causes skin irritation.
It has also been found that normal peripheral skin temperatures vary over a wide range of temperatures. Normal skin temperature not only varies from individual to individual, but also varies for a given individual under different ambient conditions. In addition, the "normal" peripheral temperature of a given individual varies during a period of approximately the first hour after the physiological monitor is first disposed on the wearer. It has been found that the monitor itself inhibits radiation heat loss from the skin area beneath the monitor, causing a gradual increase in the peripheral skin temperature under the monitor.
Further, it has been found that during sleep, peripheral blood flow (to the extremities) tends to decrease. As a result, while body core temperature does not change appreciably during sleep, the skin temperature at the extremities tends to vary, and is somewhat sensitive to ambient conditions. Accordingly, the skin temperature at, for example, the wrist, can vary as a function of the disposition of the arm, e.g. under the blankets, or exposed. Accordingly, as the ambient conditions of the extremity change due to changes in the sleeper's position, the monitoring device tends to be subject either to generation of frequent false alarms, or to the setting of an artificially low threshold temperature which requires an excessive decrease in skin temperature to generate alarm indicia. Such a situation is particularly dangerous for a diabetic incapable of perspiring due to autonomic neuropathy.
In addition, it has been found particularly difficult to manually set the temperature threshold potentiometer. As noted above, normal peripheral skin temperature varies over a wide range. The potentiometer, accordingly, must provide a range of resistances corresponding to the wide range of normal peripheral skin temperatures. The necessity of covering such a wide range of temperatures necessarily places constraints on the smallest increment of adjustment. Further, the potentiometer setting is subject to undesired changes due to accidental interaction of the adjustment thumbwheel with extrinsic objects.